Anytime personnel or equipment are located underground, such as in mines, tunnels or boreholes, a need exists for communication through-the-earth. In the past, such communications were problematic and often ineffective. While phone lines between fixed points provide some measure of communication, these fixed locations may be difficult or impossible to reach in emergency situations. Additionally, even in normal conditions, mobile communications are necessary to carry out a specific function.
Traditional radio communication utilizes frequencies above 500 kHz. These higher frequency signals typically travel only 1–10 meters into sedimentary rocks. Much greater penetration can be attained by using lower frequency transmissions, but both the transmitting and receiving ends usually require very large antennas to overcome the electrical noise inherent in typical semiconductor components. Thus, direct radio communication with underground positions has proven extremely difficult.
Other attempts at surface to underground communication have included using tunnel structures as transmission lines, usually by coupling a carrier wave onto an electrical power line (see U.S Pat. No. 4,777,652). Other systems also have been developed that use a cable with poorly confined fields, known as a leaky feeder, along with a number of repeaters to bring UHF or VHF radio service to underground areas. Another method has been to communicate with an array of transceivers connected by cables (see U.S. Pat. No. 6,370,396). This method suffers from limited range, and is easily interrupted by damage to the cables. It is therefore clear that an efficacious wireless underground communication system currently does not exist.
Previous attempts at through-the-earth communication typically have involved paging systems (see U.S. Pat. No. 6,263,189). This type system can be implemented easily in a one-way format only using existing receivers. The transmitters can be quite large, but this usually not a problem as they can be installed in a permanent location underground. In the present invention, a system has been developed that combines through-the-earth communication, low noise receivers, and digital voice compression technologies. This combination provides for two-way voice communication through-the-earth.
To understand the present invention, it is first necessary to review electromagnetic propagation of RF signals in the earth, the superconducting technology used in the receivers, and the existing audio technology. Solutions of the electromagnetic wave equations in the low frequency limit show the signal strength varies as exp(−x/δ), where δ=(2ρ/ωμ)1/2 is the depth of penetration. Here, ρ is the resistivity, ω is the angular frequency, and μ is the magnetic permeability. This relationship is illustrated in FIG. 1, where the relative magnetic permeability is assumed to be unity. Evaluating the skin depth for actual geologies is not straightforward because the resistivity is quite variable. Table 1 indicates how wide this variability is in practice.
TABLE 1Selected Resistivities (ohm-m)granite5000 × 106sea water0.2rock salt106–107coal104sandstone 35–4000limestone120–400mudstones 10–100haematite 10−1–100   galena 10−2–300   pyrite10−4–10   calcopyrite 10−4–0.1   magnetite10−2–10   pyrrhotite10−5–10−3Examining the results for a typical material, sandstone, the relationship above yield a resistivity greater than 35 ohm-meter, corresponding to a skin depth of greater than 100 meters at a frequency of 1 kHz. Conventional radio transmissions use frequencies higher than 500 kHz, but these higher frequencies are weakly ground penetrating. These results clearly imply a skin depth in common materials that would allow frequencies below 1 kHz to penetrate deeply underground. Those with skill in this art will note that many of the economically valuable ores, chalcopyrite for example, have a much lower resistivity, a fact that is used in currently established electromagnetic prospecting tools.
Geophysical noise floors were carefully studied decades ago in this spectral range. It was found that noise floors vary according to such things as the location and the weather, but is on the order of 100 fT/√Hz at 100 Hz, and decreases at higher frequencies. Importantly, this noise originates in the ionosphere. The fields that were measured at the surface of the earth in these previous studies, and are attenuated by the earth's overburden before reaching underground areas. When transmitting from a surface antenna to an underground position, the transmitted signal and the geophysical noise will be attenuated equally, maintaining the signal-to-noise ratio constant.
High temperature superconducting receivers are not expected to have noise floors below 1 fT/√Hz in the foreseeable future. Large antennas, such as those used in base stations could have a noise floor lower than the SQUIDs, but reasonable dimension limits, that is, a few meters, preclude any noise floors below those previously discussed. Thus, the naturally occurring noise easily could be negligible in all applications.
The dominant source of noise in a developed underground environment above the noise floor is the power line noise introduced by operations at the site. This power line noise consists of 60 Hz and synchronized harmonics, with the harmonics decreasing rapidly with increasing frequency. This leaves a large spectral range in which to operate.
High Temperature Superconducting (HTS) electronics have the potential to be used in a wide range of applications. This is because superconducting components usually are the lowest noise and lowest power electronics available. The basic superconducting component is the Superconducting Quantum Interference Device (SQUID), which consists of two Josephson junctions connected in parallel to form a loop. This device function by using a quantum interference effect to generate a voltage that is very sensitive to the magnetic flux threading the loop. In an actual application, a SQUID is operated with external electronic circuitry that applies current bias and flux modulation. The voltage output of the SQUID is fed to a preamplifier, and then to a phase lock circuit referenced to the modulation. The phase lock output is integrated, and superimposed out of phase onto the modulation signal. This results in the locking of the SQUID to the operating point of a specific flux. The feedback signal is then proportional to the magnetic flux flowing through the SQUID.
Of course, a communication receiver employing SQUIDs requires additional electronic circuitry that demodulates the carrier signal. Additionally, a SQUID receiver needs a cryogenic enclosure, but, using High Temperature Superconductors, the size of the receiver need not be large, as it would be if Low Temperature Superconductors (LTS) were being used. Actually, the lower cryogenic requirements of HTS allow large cost and power savings over LTS.
Traditional analog radio often uses the human voice as audio input at full bandwidth and modulates a radio frequency signal either in frequency or amplitude. In this situation, the resulting bandwidth of the RF signal equals or exceeds the bandwidth of the original audio. As previously discussed, signals that travel through the earth for substantial distances must use a carrier frequency of only a few kilohertz. This bandwidth is insufficient to carry the analog audio. However, when the analog audio is converted to a digital stream, a full audio signal typically will have a 16-bit data acquisition rate at 20 thousand samples per second. For voice audio alone, the bit rate can be reduced to 64,000 bits/sec, but this is still too high for a carrier signal of a few kilohertz.
Digital compression techniques are well developed and take advantage of the fact that the information rate in the human voice is quite low, and that much of the audio content can be discarded while retaining necessary intelligibility. Two algorithms that accomplish this compression are linear predictive coding (LPC) and multi-band excitation (MBE). LPC uses the natural resonances human speech generation to eliminate excess information in the digitized signal. MBE uses a number of frequency bands to recognize speech content. For the present invention, an MBE based algorithm implemented on a discrete digital signal processor is used. The use of audio compression allows a reduction of the bit rate to several thousand bits per second, while maintaining intelligibility. By combining audio compression techniques with digital communication on a low frequency carrier one can obtain through-the-earth transmission of voice.
As will be made clear below, the present invention is capable of transmitting intelligible audio or other data underground for a distance in excess of 100 meters. Audio also has been transmitted successfully on an axis perpendicular to the plane of the antenna, the most difficult direction to send signals.